The largest group of kinases are protein kinases, which act on and modify the activity of specific proteins. These are used extensively to transmit signals and control complex processes in cells. Up to 518 different kinases have been identified in humans. Their enormous diversity and role in signaling makes them attractive targets for drug design.

Various other kinases act on small molecules (lipids, carbohydrates, amino acids, nucleotides, and more), either for signaling or to prime them for biochemical reactions in metabolism. These are named after their substrates.